Diagnostic devices, including test plates, and, in particular, multiple well or microtitration plates, have been used for both quantitative and, especially, qualitative chemical and biological tests for decades. Various designs and configurations have proliferated as the area of enzyme immunoassays has expanded. Test devices and, particularly, plates having a plurality of wells which include microporous membrane filters, have also become routinely used in clinicallaboratories in recent times. This has resulted, at least in part, from development of cell and tissue culture techniques and assays in fields such as virology and immunology.
It is common in the clinical assay to simultaneously run a number of different tests on the same liquid sample, to run duplicate tests, or to perform the same test procedure on a number of different samples. In such instances, it is preferred to employ a multiple well filtration plate, such as a ninety-six well plate. Such test devices have advantages in that they provide a single test apparatus rather than multiple test apparatus and also provide side-by-side comparison of test results within a single device. Such plates, however, have several significant shortcomings. Many of the materials used to form at least the bottom portion of such test devices are porous in nature and permit liquid in the wells to pass through the bottom either by gravity flow or capillary action. Although such liquid loss may be permissible and even desirable in many instances, uncontrolled loss of fluid in many assays leads to inaccurate or unreliable results. This is particularly true in treating or conducting tests on living cells or tissues. In such applications, the biological material is frequently grown or maintained in media of specified composition for periods of from several hours to several days. Losses of even small volumes of liquid can in some instances alter the results drastically.
A second common problem encountered with the use of such multiple well test plates involves a phenomenon known by some as "cross-talk". Such occurrence involves the migration of liquid, sometimes in the form of a pendant drop suspended from the bottom of one well, to an adjacent well. Two causes of the type of migration known as cross-talk are (1) wicking of fluid or diffusion of solutes laterally through the membrane between adjacent wells, and (2) coalescing of pendant drops suspended below the wells. Such migration may lead to spurious results, both when the liquid removed from the wells is to be analyzed or, possibly, when liquid flows back into an adjacent well.